Apps can only print what is avalable to that app. You have to copy the document you want to print to the print app using Open In.
The exception is Print n Share. If you start it before you go to the app you want to print from, it should work.
Here is some more info on printing from the iPad that you may find useful.
Some of HP's printers are directly compatible with AirPrint the iPad.
Though the link does not reflect it Canon has recently announced that some of it's new printers will support AirPrint.
iOS: AirPrint 101
There are a couple ways around this. One is to use an printing app on the iPad. The current favorite is Print and Share. It will work with most wi-fi printers, and those that it won't wk with can be made to work with an additional program on the computer.
The second method is to load a program on the computer that will emulate an AirPrint compatible printer. Your computer must be on and have the printer available to it. The cheapest (free) is Airprint Activator. More features can be had with FingerPrint (mac and windows) and Printopia (mac only).
I use Printopia, and love it. Tim likes Fingerprint, but he has Windows, and doesn't know what he is missing.**
There is a third way. A few printers (mostly HP's again) can be set up with their own email address. PDF attachments can be sent via this address to be printed. The last review I saw said it could be quite slow (big files not recommended), and sometimes the emails got lost and never printed. The advantage was that you could literally send/print from anywhere you have an internet connection.